In optical transmission systems, it is important for the system to determine when it has failed to transmit error free data, or data with a sufficiently low number of errors to meet the design specification. It is known to count the number of errors in digital data received at the end of an optical path, and thereby calculate the bit error rate (BER).
However, when the bit error rate exceeds the design threshold, it can still be difficult to locate the source of the error whether it be in the receiver, or caused by degradation somewhere in the optical path. It becomes more desirable to be able to locate faults rapidly, when the optical path traverses an optical network containing a combination of optical amplifiers, switches, cross connects, filters, and dispersion compensators. Some types of failure can be detected and located easily. For example, some optical elements in the optical path may generate a loss of signal LOS alarm when the input signal has less power than a given threshold. However, other types of failure will not be detected by such equipment. There is a range of known causes of failure in optical networks, including optical loss, reflections, Fabry-Perot cavities, optical non-linearities, polarisation mode dispersion, optical cross talk, misconnection, and optical noise.
Another fault location method is shown in EP-A-123 132. Here telemetry telegrams are added at each regenerator in an optical transmission system. This enables faults to be located if they cause complete loss of telemetry telegrams from the far side of the location of the fault. This isolates faults to a regenerative span, but not within a single optical path.
It is also known to provide reflections at various distances along the optical path to test attenuation over sections of the optical path. EP-A-0 117 868 shows such a system, which is a form of optical time domain reflectivity (OTDR). Other OTDR systems use wave division multiplexed (WDM) signals. Such reflective systems return only a weak test signal which cannot be used to indicate a wide range of degradations.
Other known techniques are suitable for measuring particular types of degradation, but obviously cannot identify all fault conditions which can affect the optical signal and cause bit errors. For example, U.S. Pat. No. 5,513,029 (Roberts et al) discloses transmitting a low frequency dither and using it to measure the optical signal to noise ratio by detecting the depth of modulation at points in the optical path either side of an optical amplifier, to determine whether that amplifier is adding too much noise. The amplitude but not the shape of the signal is used, and so it does not indicate a wide range of degradations.
As described in EP 0 580 316, numerous analog maintenance signals are modulated onto the primary information bearing signal at different locations along the optical path. Each has a unique frequency or pattern. To determine whether there is a fault, the amplitude of each maintenance signal is monitored at the end of the optical path. There is also disclosure of injecting a maintenance signal at a slightly different wavelength to the primary information signal. Similarly, faults are indicated by an anomaly in the level of the maintenance signal detected downstream.
None of the known systems or methods can assist in locating the source of a broad range of failure mechanisms in an optical path of an optical transmission system. Counting bit errors at intermediate points in the optical path would be a very expensive option, particularly for multi gigabit systems.